How to get the dimension of multiple objects?
As Chebhou already pointed out in his comment, what people usually do when transforming a disjoint model is have an EMPTY at the centre or wherever you want it (you can use it as a pivot), and then PARENT all objects with the empty. Then you would simply transform the empty and everything should stay in the same position relative to it.
You can have a hierarchy of empties if you want to be able to move some objects separately (say wheels, doors, etc.)
I am not sure how getting the dimensions of all objects would help you solve your problem, but if that is what you want you will have to write a simple script (something like that):
for each object in selected
  print object.getDimensions()

But then you will have to go through them manually unless you don't attach it to some other script.